This invention relates generally to Magnetic Resonance Imaging (MRI) systems, and more particularly, to Radio-Frequency (RF) coils in such MRI systems.
Magnetic Resonance Imaging (MRI) or Nuclear Magnetic Resonance (NMR) imaging generally provides for the spatial discrimination of resonant interactions between RF waves and nuclei in a magnetic field. Specifically, MRI utilizes hydrogen nuclear spins of the water molecules in the human body, which are polarized by a strong, uniform, static magnetic field of a magnet. This magnetic field is commonly referred to as B0 or the main magnetic field. The magnetically polarized nuclear spins generate magnetic moments in the human body. The magnetic moments point in the direction of the main magnetic field in a steady state, but produce no useful information if these magnetic moments are not disturbed by any excitation.
The generation of NMR signals for MRI data acquisition is accomplished by exciting the magnetic moments with a uniform RF magnetic field. This RF magnetic field is commonly referred to as the B1 field or the excitation field. The B1 field is produced in the imaging region of interest by an RF transmit coil that is usually driven by a computer-controlled RF transmitter with a power amplifier. During excitation, the nuclear spin system absorbs magnetic energy and the magnetic moments precess around the direction of the main magnetic field. After excitation, the precessing magnetic moments will go through a process of Free Induction Decay (FID), releasing their absorbed energy and returning to the steady state. During FID, NMR signals are detected by the use of a receive RF coil, which is placed in the vicinity of the excited volume of the human body.
The NMR signal is the secondary electrical voltage (or current) in the receive RF coil that has been induced by the precessing magnetic moments of the human tissue. The receive RF coil can be either the transmit coil operating in a receive mode or an independent receive-only RF coil. The NMR signal is used for producing MR images by using additional pulsed magnetic gradient fields, which are generated by gradient coils integrated inside the main magnet system. The gradient fields are used to spatially encode the signals and selectively excite a specific volume of the human body. There are usually three sets of gradient coils in a standard MRI system that generate magnetic fields in the same direction of the main magnetic field, and varying linearly in the imaging volume.
In MRI, it is desirable for the excitation and reception to be spatially uniform in the imaging volume for better image uniformity. In known MRI systems, the best excitation field homogeneity is usually obtained by using a whole-body volume RF coil for transmission. The whole-body transmit coil is the largest RF coil in the system. A large coil, however, produces lower signal-to-noise ratio (SNR or S/N) if it is also used for reception, mainly because of its greater distance from the signal-generating tissues being imaged. Because a high signal-to-noise ratio is very desirable in MRI, special-purpose coils have been used for reception to enhance the S/N ratio from the volume of interest. In practice, a well-designed specialty or special-purpose RF coil has the following functional properties: high S/N ratio, good uniformity, high unloaded quality factor (Q) of the resonance circuit, and high ratio of the unloaded to loaded Q factors. Additionally, the coil should be mechanically designed to facilitate patient handling and comfort, as well as to provide a protective barrier between the patient and the RF electronics.
In order to reduce MRI data acquisition time, it is known to use faster and more powerful (e.g., greater processing power) gradient hardware. For example, parallel imaging techniques such as sensitivity encoding (SENSE) and simultaneous acquisition of spatial harmonics (SMASH) provide reduction in imaging time by using spatial information inherent in a multiple receiver coil array. In these parallel imaging techniques, multiple phase-encoded data is collected in parallel from a single phase-encoded MR signal. In operation, multiple lines of K-space data are acquired simultaneously using multiple receiver coil systems. Each component coil in an array system is characterized by a unique B1 sensitivity function. Each sensitivity function includes spatial information that may be used in the spatial encoding process. Because this information can be used to reduce the number of gradient based spatial encoding steps, imaging speed is increased.
When individuals suffer from stenotic and occlusive peripheral vascular disease, blockages or blood flow restriction develop in the arterial system. This can lead to strokes or result in amputation. When individuals develop peripheral vascular disease, a radiologist is principally interested in examining the arterial system of the individual from the heart down to the feet.
This peripheral vascular disease was at one time evaluated using X-ray technology with injected contrast agents. As MRI developed as a clinical tool, similar exams for detecting or evaluating peripheral vascular disease were performed using MR contrast agents. This technique is often called “Contrast Enhanced Multi-Station Peripheral Vascular MR Angiography.” The term multi-station applies to the procedure of moving the patient through the bore of the system in stages or stations until images from the whole length of the patient have been collected.
The resolution of the resulting images was initially poor due to the use of the large system body coil (e.g., whole-body coil). Because some of the arteries are very small in the lower legs and feet, these arteries can be hard to visualize. To improve the resolution of the MR images, phased arrays have been used. In particular, peripheral vascular (PV) coils have been used in connection with horizontal bore Mill systems. However, the coil arrays are not configured for operation in connection with open PV systems.
In horizontal systems the static magnetic field (B0) is generated in a direction such that it is parallel with the human body, with the subject lying flat. In an open system, the static magnetic field is generated in a direction transverse to the human body lying flat. Open MRI system allow much larger individuals to be imaged, including performing MRI peripheral vascular studies on these larger individuals. However, known open MRI system do not allow for PV exams to be performed. Further, although whole-body coils may be used for PV exams, the resolution of images resulting from such exams is often unacceptable, thereby making, for example, proper diagnosis impossible.